Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna subsystem and a radio subsystem including a modem, a power management section, a logical section, and a memory. In some RFID tags the power management section includes an energy storage device such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
In RFID readers, a portion of the RF signal transmitted by the transmit circuitry of the reader may couple into a receive path of the same reader as a “self-jammer” signal due to antenna reflection. Because the RF signals transmitted by the RFID tags (in response to the RF signal transmitted by the reader) are relatively weak signals, the self-jammer signal may interfere with the RF signals received from the tags causing degradation of quality or unintended operations at the reader. One approach to mitigating the self-jammer signal is attenuating a received signal at the RFID reader's front end through resistive or similar components, but that would also attenuate the already weak RFID tag signals, and is therefore an undesirable solution. Another approach is to attenuate the self-jammer signal by substantially cancelling it with a replica signal that is equal in amplitude but opposite in phase to the self-jammer signal. Such readers may generate the cancellation replica signal by combining appropriate amplitudes of two orthogonal signals that are separated in phase by 90 degrees.